1. Field of the Invention
This invention relates to a vacuum degree inspecting device for inspecting the degree of vacuum of a sealed up vessel in which contents are enclosed in a vacuum condition.
2. Description of the Prior Art
A so-called hot pack vessel which is sealed by a cap while contents therein remain in a hot condition decreases in pressure and thus produces a negative pressure in the inside thereof as the contents become cool. If the sealed condition is maintained, the top wall of the cap may be deformed in a concave or depressed condition, but on the contrary if the sealed condition fails, the top wall of the cap will not be deformed and will maintain its original convex or flat condition.
A device for inspecting the degree of vacuum of the inside of a vessel by optically inspecting whether the top wall of the vessel has a concave condition or a convex or flat condition is already known and is disclosed, for example, by Japanese Patent Publication No. 51-7063.
In the disclosed device, rays of light from a light source are changed into parallel rays having a diameter substantially equal to the diameter of the top wall of a vessel such as a can for canned food by means of a lens, and then the parallel rays are irradiated upon the top wall of the vessel by way of a half mirror. Then, the rays thus reflected from the top wall of the vessel are reflected by the half mirror and then received by a large number of photoelectric cells arranged in a particular plane. Outputs of the photoelectric cells are evaluated by a judging circuit which develops a signal representative of an insufficient degree of vacuum when it receives particular output signals from a number of the photoelectric cells greater than a preset value.
However, the conventional device has following drawbacks. (1) Rays of light from a light source are changed into parallel rays which have a diameter substantially equal to the diameter of the top wall of a cap of an object vessel and are to be irradiated upon the top wall of the vessel. Accordingly, the parallel rays will be displaced from the top wall of the vessel if the vessel is displaced out of position or inclined a little. Therefore, a very high degree of accuracy is required for positioning (centering) an object vessel, and accordingly the conventional device cannot be suitably mounted on an existing conveyor line or the like. (2) Reflected rays from the top wall of a cap of an object vessel are received by a large number of photoelectric cells, and the number of those photoelectric cells which provide particular output signals is detected in order to determine whether the vessel is to be accepted or to be rejected. Therefore, even if there is only a little fluctuation in size or configuration among top walls of caps of vessels, it will result in error in determination of acceptance or rejection.